1. Technical Field
The invention relates generally to semiconductor manufacture, and, in particular, to a method of fabricating large area, single crystalline semiconductor wafers.
2. Background Art
The production yield on integrated circuits is primarily dependent on defect density and on wafer size. Of the major semiconductor materials in use today, only silicon is available in large area (e.g., 8" diameter) low defect density boules. Gallium Arsenide is readily available in 3" diameter boules exhibiting a defect density exceeding 10.sup.4 /cm.sup.2. Indium Phosphide (InP) is available in 2" diameter boules, but defect density exceeds 10.sup.5 /cm.sup.2. Germanium (Ge) is available in defect-free boules of 2" diameter. Hexagonal (6H) Silicon Carbide (SIC) is commercially available in 1.25" diameter boules with an average defect density of 10.sup.5 /cm.sup.2, but this defect density is not uniform; large regions are virtually defect free while other regions have excessive defects. Virtually all other single crystalline semiconductors must be grown on foreign substrates. This is known as heteroepitaxy and almost always creates unwanted defects.
Silicon carbide (SIC) is known in nearly 200 different polytypes. Only hexagonal (6H) silicon carbide is commercially available in 1.25" diameter boules with an average defect density of 10.sup.5 /cm.sup.2, but this defect density is not uniform; large regions are virtually defect free while other regions have excessive defects. Cubic (3C) SiC films are available on silicon substrates, but the defect density exceeds 10.sup.8 /cm.sup.2, rendering them unsuitable for integrated circuit (IC) manufacture. There is little prospect of commercially available 6H SiC wafers exceeding 2" diameter in the near future. Since virtually all automatic wafer handling equipment requires 3" diameter and larger wafers, the prospects of using current SiC technology for large scale IC manufacture in the near future are poor.
Semiconducting diamond, although a superb material, has not been successfully grown on substrates other than natural diamond. Natural diamond substrates exceeding 1 cm diameter are rare and very expensive thus precluding widespread use of semiconducting diamond technology.
A new class of III-V compound semiconductor materials characterized by anions of nitrogen, e.g., GeN, AlN, BN, BCN.sub.2, exhibit electronic properties of great interest to the semiconductor community, but the only useful substrates for their heteroepitaxial growth are those of sapphire and SiC. In each case problems exist with respect to lattice mismatch.
Virtually all of the higher bandgap semiconductors of interest (except 6H SiC) have no substrates of like kind available for growth. The foreign substrates that are available generally have free surface energies much lower than those of the material-to-be-synthesized. As a result, the material-to-be-synthesized tends to form islands rather than deposit as a smooth two-dimensional film.
Recently it has been shown that very small (i.e., &lt;50 atomic lattice-constants in any direction) particles of a semiconductor exhibit properties different from that of their bulk counterparts. For example, the variation of melting temperature with size of CdS nanocrystals is described in a report by A. N. Goldstein, C. M. Echer, and A. P. Alivisatos, "Melting in Semiconductor Nanocrystals", SCIENCE, Vol. 256, Jun. 5, 1992, pages 425-427. Typical variations include increased bandgap, significantly reduced melting temperature, and up to a 50% reduction in dielectric constant.
As reported in a paper by D. P. Malta, J. B. Posthill, R. J. Markunas, and T. P. Humphreys entitled "Low-defect-density germanium on silicon obtained by a novel growth phenomenon", Appl. Phys. Lett. 60 (7), Feb. 17, 1992, pages 844-846, researchers at the Research Triangle Institute (R.T.I.) have demonstrated that very thin layers of germanium (e.g. &lt;50 nanometer thick) epitaxially deposited on silicon at 900.degree. C. are characterized by an interfacial melting phenomenon that appears to relieve the misfit dislocations otherwise developed by their heteroepitaxial relationship to the underlying silicon, which exhibits a lattice constant 4% smaller than the lattice constant of germanium. Unfortunately, applications of this approach appear to be limited to situations wherein the material-to-be-synthesized melts at temperatures lower than does the substrate.
As described in a paper by Y. H. Lo, W. J. Schaff, and D. Teng, entitled "EXTENDED PSEUDOMORPHIC LIMITS USING COMPLIANT SUBSTRATES" Mat. Res. Soc. Symp. Proc., Vol. 281, pages 191-196, Prof. Lo and his associates at Cornell University have demonstrated that normally lattice-mismatched semiconducting films can be grown on extremely thin substrates in such a manner that the strain is relieved in the underlying substrate rather than in the thicker heteroepitaxially grown overlayer as is the usual case for growth on substrates of normal thickness. A major problem with this approach, however, is that extremely thin substrates are not and can not be made self-supporting in large areas required for semiconductor manufacture. Nevertheless, the experiments do demonstrate that overlying layers need not necessarily exhibit misfit dislocations generated by lattice-mismatched substrates.